Supported by this grant, we have contributed to the physiology of myocardial contraction in two areas: (a) Applying the frequency response method to excised cat papillary muscle in Ba2 ion contracture, we showed that to explan the dynamic stiffness of this active state, one must consider at least two elastic and one viscous elements coupled in the Maxwell or Voigt configuration. (b) We devised and developed a new blood-perfused canine papillary muscle preparation in vivo and studied its force-length-time relation in various modes of contraction with special emphasis on the end-systolic F-L relation. In the present application we propose the following extensions of the past researches: (1) We test a hypothesis that the 3-element model is valid for excised kitten papillary muscles in tetanic contraction by ryanodine, in potassium contracture, or in contracture after EGTA treatment, despite the different mechanisms behind these tonic contractions. Both sinusoidal and stepwise length perturbations will be used for the test. (2) We investigate the non-uniform shortening between the core vs. surface of our canine papillary muscle preparation twitching in vivo, by a modification of the pin marker method. (3) Using the original version of the paired pin marker method in excised kitten papillary muscle, we will reinvestigate the instantaneous relation of force to middle segment length (rather than the whole muscle length). To characterize the time-varying nature of the F-L relation we will use a new pulse response method, i.e., length or velocity pulse perturbations injected at various phases of the contraction cycle, with the aid of computer control of experimental protocol and on-line processing of force response data. (4) We will develop a laser diffraction analysis system for sarcomere kinematics to assess the validity of our three element model.